Research and development on high-density optical discs have been actively carried out in recent years, DVDs have been recently generally used and optical discs are establishing a position as important information recording media. In DVDs, a method for demodulating digital data by a digital signal processing using PRML (Partial Response Maximum Likelihood) effective for high density recording and reproduction in a linear direction has been brought in to improve the reproduction quality of media capable of high-density recording.
On the other hand, for a reproduction signal reproduced from a DVD having incompletely shaped marks recorded due to a recording performance and the like, reproduction quality may be degraded in the PRML signal processing as compared with a level discrimination binary processing for binary discrimination using a slice level at which a balance of recorded codes is satisfied.
Thus, an optical disc device for measuring a mark distortion rate for a mark pattern with a long recording width in a DVD and switching a signal processing between a PRML signal processing and a level discrimination binary processing based on the mark distortion rate is disclosed, for example, in patent literature 1. In this optical disc device, reproduction quality unaffected by mark distortion can be obtained in the reproduction of a DVD. Further, patent literature 2 discloses to satisfactorily reproduce information from an optical disc having information recorded at high density using two types of waveform equalizing circuits having different equalization characteristics.
BDs (Blu-ray Discs) such as BD-Rs (Blu-ray Disc Recordables) formed with organic dye recording films by spin coating and BD-REs (Blu-ray Disc Rewritables) formed with recording films made of phase-change materials by deposition have been proposed and put to practical use as a tendency toward higher density. Out of these BDs, those formed with organic dye recording films by spin coating easily enable a cost reduction, wherefore they are predicted to enter the mainstream of inexpensive high-density optical recording media.
FIG. 12 is a diagram showing the structure of a BD formed with an organic dye recording film by spin coating. In the BD shown in FIG. 12, a reflection film 53 is formed on an injection molded substrate 54 by sputtering or the like, an organic dye recording layer 52 is formed on the reflection film 53 by spin coating and a transparent film 51 is formed on the recording layer 52. This BD has a groove track (on-groove) OG and a land track (in-groove) IG, and a laser beam 50 is irradiated to the recording layer 52 via the transparent layer 51 to record information on the land track IG (see, for example, patent literature 3).
The BD shown in FIG. 12 has no HtoL characteristic in which the reflectance of a recorded area is lower than that of an unrecorded area, but has an LtoH characteristic in which the reflectance of a recorded area is higher than that of an unrecorded area. In order to ensure a sufficient reflectance, the depth of a track groove (difference between the height of the groove track OG and that of the land track IG) is set to be smaller than the one in an optical recording medium having an HtoL characteristic (see, for example, patent literature 4).
As described above, in the BD having the LtoH characteristic in which the reflectance of the recorded area is higher than that of the unrecorded area and having information recorded on the land track, the depth of the track groove is small, wherefore heat is easily trapped and very unbalanced recording is likely to be performed as compared to BDs with other combinations (BD having the LtoH characteristic and having information recorded on a groove track, BD having the HtoL characteristic and having information recorded on the groove track or the land track). Thus, tear-shaped or M-shaped marks (recorded areas) as described below are more likely to be recorded, thereby degrading the recording quality.
FIG. 13 is a diagram showing an example of the tear-shaped mark and FIG. 14 is a diagram showing an example of the M-shaped mark. Upon reproducing the tear-shaped mark MD shown in FIG. 13, a reproduction signal SD has a large amplitude in the front half, but has a small amplitude in the rear half. Upon reproducing the M-shaped mark MM shown in FIG. 14, a reproduction signal SB has a large amplitude at a front side, has a small amplitude in the middle and has a large amplitude again at a rear side.
Since a shortest recording mark is very small and subject to intersymbol interference during reproduction in a BD, reproduction by the level discrimination binary processing is difficult. As a result, the level discrimination binary processing used in DVDs cannot be used and the PRML signal processing needs to be used. However, in the above BD having the LtoH characteristic and having information recorded on the land track, a signal with a very large waveform distortion is reproduced as compared with other BDs. Thus, errors frequently occur and signals with good reproduction quality cannot be obtained if the conventional PRML signal processing, which cannot deliver sufficient performance to a reproduction signal with a large waveform distortion, is used as it is.    Patent Literature 1:    Japanese Unexamined Patent Publication No. 2005-93033    Patent Literature 2:    Japanese Unexamined Patent Publication No. 2002-230904    Patent Literature 3:    A1 Pamphlet of International Publication WO 2006/006458    Patent Literature 4:    A1 Pamphlet of International Publication WO 2006/049006